I get it but regard it as a kind of rule of thumb that works out. The math is favorable. 9' table has a 50" surface width. 14.5 ft is 174". Minus playing surface = 124". Divide by the two sides and you're at 62" clearance for a 4" back swing if you're shooting straight off the rail. Tight, maybe, but how much back swing are you ever using when shooting off the rail? I'm either jacked up or taking a careful short level stroke trying not to miscue.
I'm fortunate that my room is 15'6". But on the wall I wanted to be close to, which is the long side, I centered the table on 19' relative to that wall and there's plenty of stroke room on that side.
Now, actually using a room that is 14.5x19 means having nothing in that room except a pool table. My room is something like 22"6' long and I'm going to have to be creative getting a work bench and seating in there.